In recent years, roller chains have come into increasing use, as a result of the demand for high load capacity, high speed operation and maintenance free operation. For these reasons, roller chains have displaced toothed belts, which were previously commonly used in automotive timing drives.
As shown in FIGS. 9(a)-9(c), a conventional roller chain 500 for a timing transmission comprises an inner link 520 (FIG. 9(b)) and an outer link 540 (FIG. 9(c). In the inner link 520, both ends of each of two cylindrical bushings 522 fit in bushing holes 526 in a pair of inner plates 524, and rollers 528 are rotatable on the bushings 522. In the outer link 540, two pins 542 are secured to pin holes 546 in a pair of outer plates 544, disposed on the outsides of the pair of inner plates 524. The pins 542 extend through the bushings, and the pins and bushings are relatively rotatable. The roller chain meshes with sprockets (not shown in FIGS. 9(a)-9(c).
A conventional roller chain transmission utilizes a so-called “A series” roller chain, which satisfies the relationships 0.591≦D/P≦0.640, and 0.281≦d/P≦0.377, and a “B series” roller chain, which satisfies the relationships 0.575≦D/P≦0.670, and 0.288≦d/P≦0.389, according to JIS B 1801, D being the outer diameter of the roller, d being the outer diameter of the pin, and P being the pitch of the roller chain.
These standards have generally been adhered to because it has been considered that, when the ratio D/P is too high, the widths of the sprocket teeth become excessively small and insufficiently strong. On the other hand, when the ratio D/P is too low, the outer diameter d of the pins, which extend through the bushings, is decreased, which leads to reduction in the strength of the chain. Thus, among the B series roller chains, a chain having a pitch of 9.525 mm, a roller outer diameter of 6.35 mm and a pin outer diameter of 3.28 mm, that is, an “06B” chain according to ISO standards, has been used often as a timing transmission medium for an automobile engine.
Sprockets used with these roller chains, have standard, so-called “S” or “U” tooth forms specified in JIS and ISO standards, designed to achieve smooth engagement with, and disengagement from, the chain and efficient transmission of torque to a shaft.
It has been reported that, when the above-mentioned conventional roller chains were used as timing chains in an automobile engine under high loads, for example 4 kN, some of the chains fail to exhibit the expected durability. Furthermore, as a result of user demands, improved quietness, miniaturization, and weight saving, have been required for roller chain transmissions in automobile engines.
As a result of intensive study of the above problems, we have found that the “elongation—wear resistance” of a roller chain, which may be within the allowable range in other applications, becomes critical when a roller chain is used as a camshaft drive chain for precision timing in an automobile engine, and unexpectedly high loads are applied to the chain link connected pins at higher engine speeds. Thus, it has become clear that the size ratios (D/P, d/P) of the roller chain pitch P, the outer diameter D of the roller, and the outer diameter d of the pin, which are conventionally set on the basis of size balances, are not necessarily optimum values.
Further, it has become clear that the specified standard tooth forms of conventional sprockets are not necessarily optimum from the standpoint of quietness and wear resistance of the sprockets.
Accordingly, the objects of the invention are to solve the above-described problems of conventional roller chain transmission devices by optimizing the shapes and sizes of the roller chain and sprocket; to provide a roller chain transmission which enhances the wear resistance, strength and quietness; and, in particular, to provide a roller chain transmission which exhibits sufficient endurance and quietness even when used as a timing drive in an automobile engine operated at high speed.